It is known that such nozzles may be fixed or may exhibit a very low deformability but that, for certain applications, requiring, for example, orientable nozzles, their movements may be relatively great. A nozzle exhibits, in general, a substantially frustoconical form. At rest, the nozzle is in a position such that its axis is substantially parallel to the axis of the propulsion unit. The movements of the nozzle consist principally of an inclination of its axis in relation to the axis of the propulsion unit. The nozzle may likewise become deformed slightly under the effect of the hot gases which circulate therein. The invention relates to a heat protection which can be used irrespective of the magnitude of the deformations or displacements of the nozzle.
It is further known that a rocket, just like a missile, is capable of being loaded on board an aircraft, a ship or a submarine. In such vehicles, the available space is measured and it is desired to have available the maximum of elements loaded in the minimum of space, without adversely affecting their integrity and without nevertheless impairing their effectiveness. Accordingly, it is beneficial to have an on-board projectile, rocket or missile, which has a particularly small length while still being capable of accomplishing the contemplated mission, with increased performance levels, for the same overall length of the projectile in the active phase. This result may be obtained by equipping the propulsion unit of the on-board projectile with a nozzle comprising an unfoldable divergent section. The divergent section is unfolded after the ignition of the propulsion unit when the projectile is in free flight and is no longer subject to limitations of length, in order to contribute to increasing the pressure reduction ratio of the gases in the nozzle and thus to improve the propulsion yield.
It is thus possible to refer to French Patent No. 2,422,831, which relates to a propulsion unit nozzle with an unfoldable divergent section, the divergent section being constituted by a succession of rigid rings connected to one another and to the nozzle body by flexible rings. In the folded position of the divergent section, these flexible and rigid rings are juxtaposed about one another and about the nozzle body according to an accordion profile. In the unfolded position of the divergent section, they are aligned one behind the other according to the profile desired for the divergent section in the active configuration.
It is likewise possible to refer to French Patent No. 2,457,390, which likewise relates to an unfoldable divergent section of a nozzle for a rocket propulsion unit. This divergent section comprises a set of elemental individual panels of refractory material which are articulated to one another and in relation to a divergent section fixed upstream part close to the neck of the nozzle. The articulated panels are distributed in such a manner as to define at least two crowns or frusta of a cone which are adjacent and successive and to be able to be placed in a first, folded position and in a second, unfolded position.
More specifically, the invention relates to a heat protection which is capable of being constructed both for nozzles constructed in one single piece and for nozzles which are at least partially unfoldable or comprise a divergent section which is at least partially unfoldable.
Numerous heat protection devices are already known, some of these being intended more particularly for the elements placed to the rear of the propulsion units. They may consist of an individual protection of each element which it is desired to protect from the heat fluxes emanating from the nozzle.
Thus, each structural element, for example the skirt or the base of the propulsion unit, as well as each system placed within the rear zone of the propulsion unit, may be equipped with a rigid insulating shell protecting it from the heat flux.
The patent FR-2,489,812 describes a method for the production of shaped parts which are capable of withstanding relatively great thermal shocks as well as high temperatures, in the order of 1600.degree. C. These parts are constructed from a substrate formed of inorganic fibers rigidified by at least one inorganic binder and possibly an organic binder, the substrate being constructed by the technique of suction molding and the whole having undergone a treatment of reinforcement by a resin. The shaped parts which are obtained are resistant to delamination and to bursting under the effect of an excess pressure or of a reduced pressure of the gas and furthermore exhibit a good actual mechanical stability and an excellent machinability.
This technique of individual protection of each structural element and of each system exhibits numerous disadvantages.
First of all, it requires the use of a surface and thus of a mass of heat protection material which are very great.
Furthermore, these protective parts or shells are fixed mechanically or by adhesive bonding onto each structural element or each system. This means that only the characteristics of thermal insulation of the materials making up these shells or these parts are used. Furthermore, the adhesive bonding of the heat protection elements on the systems gives rise to numerous difficulties in the course of the maintenance of these systems. In fact, after the demounting of the systems, it is necessary in particular to undertake the removal of the protections and the descaling of the adhesive bonding surfaces.
Although the complexity of the construction of the protective parts is a function of the technique employed, it is nevertheless true that a specific tooling is necessary for the construction of each protective part intended for a particular structural element or system. Thus, the method described in the patent FR-2,489,812 requires the prior construction, for each particular protective part, of an suction mold intended to form this part.
Finally, it is found that these individual techniques of heat protection require numerous manual operations, especially operations of molding, cutting out, adjustment or again of adhesive bonding. Thus, these techniques are relatively costly.
Accordingly, it is then possible to contemplate a heat protection permitting the global protection of a first zone from the heat fluxes emanating from a second zone, which may be subjected to sudden and large variations of temperature and of pressure.
Thus, the patent U.S. Pat. No. 4,324,167 describes a device providing, between the ignition and the launch of a rocket, a seal between the launcher tube and the rear of the rocket. This device thus protects the launcher tube from the high-temperature gases which are generated by the rocket as soon as it has been ignited. This protection comprises a support screen and a flexible protection which are superposed, placed at the lower part of the rocket. In the course of the ignition of the rocket, a pressure is exerted against the flexible protection, which is deformed in order to come into contact with the rocket and to form a seal. When the rocket takes off, it fractures the seal, the exhaust gases escaping from the nozzle, then causing the deformation of the protection and the increasing of the gas flux through the latter.
This heat protection is effective to protect the launcher tube in which a missile or a rocket is placed from the heat fluxes emitted by the nozzles. However, this protection has no effect on the systems placed within the rear zones of the propulsion unit.
It is, however, possible to contemplate heat insulation by virtue of a heat screen ensuring a global heat protection of either an entire zone comprising systems or a fraction of this zone corresponding to a particular system which is sensitive to the heat flux.
A global heat protection has advantages. In fact, its construction requires less mold and tooling than the technique of individual heat protection. Accordingly, it is less costly than the latter. Furthermore, access to these systems which it protects is easier and their maintenance is facilitated thereby. In particular, the operations of descaling of the systems in order to remove the residues of adhesive and of insulating material, resulting from the adhesive bonding of the heat protection parts on the systems, are eliminated. Finally, the surface and thus the mass of the heat protection materials required for the construction of such a global heat protection are far smaller than those involved in the construction of individual heat protection parts which would be required to protect the same systems. It can be found that the mass gain is 25 to 30%.
The global heat protection as described in the patent U.S. Pat. No. 4,324,167 does, however, exhibit disadvantages. Thus, it is found that it is pressure-sealed. Consequently, it gives rise to large stresses on the carrying structures to which it is fixed. This demands the reinforcement of these carrying structures and thus involves an increase in their mass, thereby reducing the performances of the projectile on which the heat protection is constructed.
Heat protections are known which are rigid and pressure-permeable. However, these exhibit only a relatively poor mechanical stability. Reference may be made, once again, to the patent FR-2,489,812, which describes heat protection parts and materials exhibiting a relatively high porosity and which thus resist bursting under the effects of excess pressures or of reduced pressures of gases. However, these parts or materials do not exhibit any good mechanical characteristics and require carrying structures such as frames. This involves an increase in the mass of the heat protection constructed; as previously, this reduces the performances of the projectile on which the heat protection is mounted.
Finally, such heat protections are used for nozzles which are fixed or which exhibit a very low deformability, but cannot be used for nozzles, the movements of which are relatively large.
Accordingly, it is possible to contemplate a rigid device for heat protection of elements subjected to the heat fluxes emitted by hot-gas generators, which is permeable to the excess pressures created by the ignition of the generators in order to limit the stresses on the carrying structures permitting the withstanding of high heat fluxes and adaptation to large deformations of these generators. In the case where the hot-gas generator is constituted by the nozzle of a rocket or missile propulsion unit, this device may, for example, consist of a heat screen composed of two self-supporting and non-integral parts, one part being fixed to the nozzle and the other part to the skirt of the propulsion unit. A baffle being formed between these two parts, the screen is permeable to the pressure while ensuring an effective protection of a first zone at the rear of the propulsion unit, which zone is situated between the case of the propulsion unit and the protection device, in relation to a second zone situated between the protection device and the ambient environment.
However, such a heat protection device cannot be used for nozzles comprising a divergent section which is at least partially unfoldable.
This device also exhibits disadvantages. In fact, each part making up the screen can be constructed of a material which is either sealed or pressure-permeable.
In the first case, large stresses are exerted on the structures to which it is fixed; this requires the placing of reinforcements in position. In the second case, the material does not exhibit adequate mechanical stability and it is necessary to provide carrying structures. Accordingly, it is found that, in both cases, the mass of the heat protection device is necessarily increased; this reduces the performances of the projectile on which it is mounted.